Multi-physical quantity responsive multilayer inverse opal hydrogel film and preparation method thereof

Abstract

The application discloses a kind of multilayer inverse opal hydrogel film of multi-physical quantity response and preparation method thereof, belong to structural color film technical field, the structure of the film is inverse opal photonic crystal structure, by the periodic arrangement air hole and temperature-sensitive solvent double-responsive hydrogel P (NIPAM-AM) Composition, air hole arrangement is face-centered hexagonal close-packed.The film is divided into upper and lower two-layer structure and is composed of air hole and filling material, wherein air hole is periodic superposition, the air hole radius of upper structure is at least 20 nm greater than the air hole radius of lower structure, the hydrogel film of the application has the characteristics such as composite anti-fake and multi-response, using multilayer structure and composite material can make hydrogel film realize monochromatic and superposition color control under multiple physical quantity conditions, improve the technical level of structural color film in anti-fake aspect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of photonic crystals, and particularly relates to a multi-physical quantity response multilayer inverse opal hydrogel film and a preparation method thereof. BACKGROUND

[0002] In nature, color plays an extremely important role in the process of biological information transmission. According to the mechanism of coloration, the colors in nature can be divided into chemical colors and physical colors. Chemical color is due to the fact that the chemicals on the surface of an object absorb light of a specific wavelength and reflect the rest of the light, thus showing color, among which dyes and pigments are common chemical colors. Physical color, also known as structural color, is the color produced by the refraction, reflection, diffraction or interference of light caused by the microstructure of an object. Compared with chemical colors presented by pigments and dyes, structural colors are attracting more and more attention from the scientific and engineering communities due to their high brightness, high saturation, never-fading and environmental protection characteristics.

[0003] With the in-depth study of structural colors, to meet the various needs of human beings, the use of materials with stimulus responsiveness to prepare responsive photonic crystal structural color materials has attracted the attention of a large number of researchers. At present, functionalized stimulus-responsive structural color materials provide an efficient and convenient information processing means for people, simplify the way of people's cognition of external stimuli, and gradually appear in people's daily life. The existing structural color materials can already replace related instruments and equipment in many fields to complete the real-time monitoring and sensing of some important information, and these materials have been widely applied to the fields of anti-counterfeiting and sensing, saving a lot of resources while changing the traditional analog-digital conversion monitoring method.

[0004] Based on the comprehensive analysis of the research results at home and abroad, the existing research mainly focuses on the following points:

[0005] (1) Fully exert the physicochemical properties of polymer materials to prepare photonic crystal structural color materials with excellent characteristics. Bin Dong et al. of Dalian University of Nationalities proposed a humidity-responsive composite inverse opal photonic crystal film and a preparation method thereof (CN112225939B). The patent relates to the construction and preparation method of a stealth encryption information photonic crystal film with stable mechanical properties and response color changing performance to polar and non-polar solvents, which can prepare a metal luster structural color film with stealth encryption information and realize local structural color response color changing decryption under the stimulation of different polar solvents.

[0006] (2) From the demand to innovate the microstructure of structural color hydrogel, construct high-strength hydrogel with stimulus responsiveness, and further expand the application to anti-counterfeiting, display and other fields. Zhou Jinming et al. of Hebei Normal University proposed a physical unclonable structural color anti-counterfeiting label (CN110766119B) with multiple anti-counterfeiting modes. The anti-counterfeiting label is composed of a patterned surface layer with disordered optical structure, a water-transparent white color intermediate layer and a black background bottom layer, successfully realizing a triple physical unclonable anti-counterfeiting mode.

[0007] At present, the design and preparation based on photonic bandgap structure to realize the presentation and regulation of structural color have been realized in various forms. The method for presenting structural color is becoming simpler and simpler, and the color range of structural color is becoming wider and wider. However, most of the structural color hydrogel films for anti-counterfeiting at present focus on single-sided pattern color regulation (such as Site L, Yang H, Yangjie L, et al. Structural colored water rewritable paper enabled by assembled graphene laminates / SiO2 amorphous colloidal arrays hierarchical structure; Yue W, Ruikang S, Yaqun H, et al. Ultrathin photonic crystal film with supersensitive thermochromism in air), or double-sided pattern color regulation can only be regulated by a single physical field, and the reflection peak intensity in the reflection spectrum is not high (mostly less than 0.7) (such as Yong Q, Lin C, Wenbin N, et al. New Encryption Strategy of Photonic Crystals with Bilayer Inverse Heterostructure Guided from Transparency Response.). At the same time, there are contradictions in the preparation process of structural color hydrogel film: to realize the multi-physical quantity response of hydrogel film, the ratio of added reaction monomers needs to be increased, which will affect the chemical bond of hydrogel film, and further affect the responsiveness of hydrogel film. In addition, it is also a difficult problem to be solved to maintain high intensity of reflection peak while realizing single color and superimposed color regulation of hydrogel through multiple physical quantities. SUMMARY

[0008] The technical problem to be solved by the present application is to provide a multi-physical quantity response multilayer inverse opal structure hydrogel film, which has high symmetry and good stability, so that it has excellent optical properties and good mechanical properties.

[0009] To solve the above technical problems, the technical scheme adopted by the present application is:

[0010] A multi-physical quantity response multilayer inverse opal hydrogel film, the structure of the film is an inverse opal photonic crystal structure, which is composed of periodically arranged air holes and temperature-sensitive solvent double-responsive hydrogel poly(N-isopropyl acrylamide-co-acrylamide) (P(NIPAM-AM)), and the air holes are arranged in a face-centered hexagonal close-packed manner. The film structure is divided into upper and lower layers, both of which are composed of air holes and filling materials, and the air holes are periodically superimposed, and the air hole radius of the upper layer structure is at least 20nm larger than that of the lower layer structure.

[0011] The further improvement of the technical scheme of the present application is that the air hole radius of the upper layer structure is 135nm-165nm, the air hole radius of the lower layer structure is 105nm-145nm, and the difference between the air hole radii of the upper and lower layers is at least 20nm.

[0012] The further improvement of the technical scheme of the present application is that the number of layers of the air holes of the upper layer structure is 4-7 layers, and the number of layers of the air holes of the lower layer structure is 5-9 layers, and the number of layers of the air holes of the upper layer structure and the lower layer structure is not the same.

[0013] A preparation method of a multi-physical quantity response multilayer inverse opal structure hydrogel film:

[0014] Step 1: Prepare monodisperse nanospheres, disperse dry monodisperse nanospheres of the same particle size into a trisodium citrate solution and stir, then centrifuge, ultrasonic, and dry the dispersed solution to obtain surface-modified nanospheres;

[0015] Step 2: Disperse the surface-modified nanospheres prepared in step 1 in a solvent containing anhydrous ethanol and dimethyl sulfoxide (DMSO), then sequentially add acrylamide (AM), N-isopropyl acrylamide (NIPAM), photoinitiator (Irg.124), and methylene bisacrylamide (BIS) into the mixed solution containing surface-modified nanospheres, anhydrous ethanol, and DMSO, and ultrasonically disperse to obtain a precursor solution;

[0016] Step 3: Pour the precursor solution obtained in step 2 into a 3D printed rectangular box mold, then use a UV lamp for polymerization to obtain a P(NIPAM-AM) opal structure hydrogel film;

[0017] Step 4: By repeating step 3 using rectangular box molds of different heights, P(NIPAM-AM) opal structure hydrogel films of different thicknesses can be obtained.

[0018] Step 5: Take dry monodisperse nanospheres of different particle sizes and repeat steps 1, 2, 3, and 4 to prepare P(NIPAM-AM) opal structure hydrogel films with different particle sizes and different film thicknesses.

[0019] Step 6: Take two P(NIPAM-AM) opal structure hydrogel films with different particle sizes of nanospheres from Step 5, and the two hydrogel films have different thicknesses. Add a crosslinking agent solution between the two hydrogel films, cure them into a composite film using a UV lamp, and then etch the composite film in a hydrofluoric acid solution to obtain a multilayer P(NIPAM-AM) inverse opal structure hydrogel film.

[0020] A further improvement of the technical solution of the present invention is that the monodisperse nanospheres in step 1 are one of silicon dioxide (SiO2), titanium dioxide (TiO2), zinc oxide (ZnO), polystyrene (PS), polymethyl methacrylate (PMMA) and cadmium sulfide (CdS) nanospheres.

[0021] A further improvement of the technical solution of the present invention is that: in step 2, the volume ratio of anhydrous ethanol to DMSO is 1:4.5-5.0, the mass-volume ratio (mg / μL) of the surface-modified nanospheres to the mixed solvent of anhydrous ethanol and DMSO is 1:9-10, and the mass ratio of the surface-modified nanospheres, AM, NIPAM, Irg.124, and BIS is 11.1-12.1:29.6-31.6:148.1-151.3:3.7-4.7:1-1.5.

[0022] A further improvement to the technical solution of the present invention is that: in step 1, the mass fraction of the trisodium citrate solution is 10%, and the mixture is stirred for 2-4 hours; in step 2, the mixture is ultrasonically dispersed at an ambient temperature of 25°C for 1-2 hours; and in step 3, the mixture is polymerized under an ultraviolet lamp for 30-60 seconds.

[0023] A further improvement to the technical solution of the present invention is that: in step 4, the height of the 3D printed rectangular box mold is in the range of 0.5 to 0.9 mm, and in step 6, it is etched in hydrofluoric acid solution for 10 min.

[0024] The technological advancements achieved by this invention due to the adoption of the above technical solutions are as follows:

[0025] This invention relates to a multilayer inverse opal hydrogel film with multi-physical response. The dual-to-single photon bandgap is controlled through ingenious design of the upper and lower layer structures and air pores. Simultaneously, an innovative preparation process optimizes the mass ratio of the reactive monomers. Because NIPAM contains hydrophobic isopropyl groups, its hydrophilicity is weaker than AM. AM, being hydrophilic and abundant in the polymer network, enhances the mechanical properties of the hydrogel film through hydrogen bonding with a large amount of water. Therefore, as the proportion of NIPAM increases, the overall hydrogen bonding in the system weakens, making the hydrogel film more susceptible to damage under external physical fields. Furthermore, the mass of Irg.124 and BIS also affects the mechanical properties of the hydrogel. Therefore, considering the characteristics of the reactive monomers, the preparation process of this invention determines the mass ratio of surface-modified nanospheres, AM, NIPAM, Irg.124, and BIS, effectively solving the problem of the hydrogel film's structural susceptibility to damage under various physical fields.

[0026] Temperature Response: Due to the multilayered inverse opal structure of the P(NIPAM-AM) hydrogel, a primary reflection peak and a secondary reflection peak appear in the spectrum when no external physical field is applied, resulting in a superimposed color in the hydrogel film. Since NIPAM in P(NIPAM-AM) is a thermosensitive material, the temperature response range of the hydrogel film is 32–56℃. Within this range, as the temperature increases, the hydrogel film gradually shrinks, and the spacing between the microspheres gradually decreases. According to the Bragg-Snell law, the color of the hydrogel film gradually shifts to blue, which is reflected in the spectrum as the wavelength corresponding to the location of the photonic crystal reflection peak gradually decreases. When the hydrogel film is heated to 32℃, the upper photonic crystal structure shrinks with increasing temperature, causing a blue shift in the reflection peak, resulting in a color change. Secondly, as the entire hydrogel film is heated, the reflection peak of the lower photonic crystal structure also undergoes a blue shift, building upon the blue shift of the upper peak. Since the lower peak has a wider half-width at half-maximum (FWHM), it overlaps with the upper peak after its blue shift, resulting in a single reflection peak in the reflection spectrum, causing another color change. In summary, the multilayer inverse opal structure hydrogel film exhibits two color changes throughout its temperature response, achieving a switch from a two-photon bandgap to a single-photon bandgap, i.e., a color transition from superimposed colors to monochromatic colors. This further enriches the color modulation capabilities of hydrogel films. During the color change process, the intensity of the photonic crystal's main reflection peak is consistently greater than 0.8, reaching a maximum of 0.99, and no significant loss in reflection peak intensity is observed during the hydrogel film's temperature response. Traditional inverse opal-structured thermosensitive hydrogels, when placed at a fixed temperature, will only exhibit one color change in the hydrogel film, and the intensity of the reflection peak is not high.

[0027] Solvent Response: Without an applied external physical field, the basic principle is the same as the temperature response, and the hydrogel film exhibits a superimposed color. When the upper inverse opal structure is completely wetted with anhydrous ethanol of a concentration greater than 30 wt%, the hydrogel film undergoes a swelling effect. According to the Bragg-Snell law, the reflection peak generated by the upper inverse opal structure redshifts to the near-infrared region, the color of the upper inverse opal structure disappears, and the upper layer of the film becomes approximately transparent, with almost no scattering effect on the color of the lower layer. Furthermore, after ethanol wetting, the effective refractive index of the entire film increases, thus enhancing and redshifting the reflection peak of the lower photonic crystal. When the entire hydrogel film is wetted, the entire hydrogel film becomes transparent. This hydrogel film exhibits solvent response characteristics and can achieve a switch from a two-photon bandgap to a single-photon bandgap, i.e., a color transition from superimposed to monochromatic, further enriching the color control of the hydrogel film. During the solvent response process, the intensity of the main reflection peak remains greater than 0.82, reaching a maximum of 0.98. No significant loss of reflection peak intensity was observed during the solvent response process of the hydrogel film.

[0028] Simultaneous Solvent and Temperature Response: Without an applied external physical field, the basic principle is the same as the temperature and solvent responses, resulting in a superimposed color in the hydrogel film. Heating the hydrogel film to 32°C and simultaneously wetting it with anhydrous ethanol at a concentration greater than 30 wt% initially causes a blue shift in the reflection peak of the upper photonic crystal structure as the temperature increases. However, when the upper inverse opal structure is fully wetted, its reflection peak red-shifts. The combined effect of these two factors results in a blue shift in the hydrogel film's color. Finally, after the hydrogel film has fully responded, only one reflection peak appears in the reflection spectrum, exhibiting a slight blue shift compared to the initial state. Considering the entire temperature and solvent dual-response process of the multilayer inverse opal structure hydrogel film, two color changes are observed. The reflection spectrum indicates that this composite hydrogel film can respond simultaneously to temperature and solvent, with temperature having a greater impact on color than solvent response, potentially enriching the color modulation capabilities of the hydrogel film. Throughout the dual-physical-quantity response process, the intensity of the main reflection peak remains greater than 0.9, reaching a maximum of 0.99. The intensity loss of the reflection peak is minimal during the dual physical quantity response process of the hydrogel film. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the multilayer inverse opal hydrogel film structure with multi-physical quantity response of the present invention.

[0030] Figure 2 This is a graph showing the heating temperature response of the hydrogel film of the present invention, which has 4 and 7 layers in the upper and lower layers and air pore radii of 105 nm and 140 nm respectively.

[0031] Figure 3This is a graph showing the heating temperature response of the hydrogel film of the present invention, which has 5 and 8 layers in the upper and lower layers and air pore radii of 105 nm and 140 nm respectively.

[0032] Figure 4 This is a graph showing the heating temperature response of the hydrogel film of the present invention, which has 6 and 9 layers in the upper and lower layers and air pore radii of 105 nm and 140 nm respectively.

[0033] Figure 5 This is a graph showing the heating temperature response of the hydrogel film of the present invention, which has 4 and 7 layers in the upper and lower layers and air pore radii of 145 nm and 165 nm respectively.

[0034] Figure 6 This is a graph showing the heating temperature response of the hydrogel film of the present invention, which has 5 and 8 layers in the upper and lower layers and air pore radii of 145 nm and 165 nm respectively.

[0035] Figure 7 This is a graph showing the heating temperature response of the hydrogel film of the present invention, which has 6 and 9 layers in the upper and lower layers and air pore radii of 145 nm and 165 nm respectively.

[0036] Figure 8 This is a graph showing the ethanol solvent response results of the hydrogel film of the present invention, which has 4 and 7 layers on the top and bottom layers respectively, and air pore radii of 105 nm and 140 nm.

[0037] Figure 9 This is a graph showing the ethanol solvent response results of the hydrogel film of the present invention with 5 and 8 layers on the upper and lower layers, and air pore radii of 105 nm and 140 nm respectively.

[0038] Figure 10 This is a graph showing the ethanol solvent response results of the hydrogel film of the present invention with 6 and 9 upper and lower layers and air pore radii of 105 nm and 140 nm respectively.

[0039] Figure 11 This is a graph showing the ethanol solvent response results of the hydrogel film of the present invention with 4 and 7 upper and lower layers and air pore radii of 145 nm and 165 nm respectively.

[0040] Figure 12 This is a graph showing the ethanol solvent response results of the hydrogel film of the present invention with 5 and 8 layers on the upper and lower layers, and air pore radii of 145 nm and 165 nm respectively.

[0041] Figure 13 This is a graph showing the ethanol solvent response results of the hydrogel film of the present invention with 6 and 9 upper and lower layers and air pore radii of 145 nm and 165 nm respectively.

[0042] Figure 14 This is a graph showing the response of the hydrogel film of the present invention, which has 4 and 7 layers on the top and bottom layers respectively, and air pore radii of 105 nm and 140 nm, to ethanol solvent and heating temperature.

[0043] Figure 15 This is a graph showing the response of the hydrogel film of the present invention, which has 5 and 8 layers on the top and bottom layers respectively, and air pore radii of 105 nm and 140 nm, to ethanol solvent and heating temperature.

[0044] Figure 16 This is a graph showing the response of the hydrogel film of the present invention, which has 6 and 9 layers on the top and bottom layers respectively, and air pore radii of 105 nm and 140 nm, to ethanol solvent and heating temperature.

[0045] Figure 17 This is a graph showing the response of the hydrogel film of the present invention, which has 4 and 7 layers on the top and bottom layers respectively, and air pore radii of 145 nm and 165 nm, to ethanol solvent and heating temperature.

[0046] Figure 18 This is a graph showing the response of the hydrogel film of the present invention, which has 5 and 8 layers on the top and bottom layers respectively, and air pore radii of 145 nm and 165 nm respectively, to ethanol solvent and heating temperature.

[0047] Figure 19 The graph shows the response of the hydrogel film of the present invention, which has 6 and 9 layers on the top and bottom, respectively, and air pore radii of 145 nm and 165 nm, to both ethanol solvent and heating temperature. Detailed Implementation

[0048] The present invention will be further described in detail below with reference to embodiments:

[0049] like Figure 1 This paper presents a multilayer inverse opal hydrogel film with multiple physical quantity responses. The film structure has the following characteristics:

[0050] The thin film structure is an inverse opal photonic crystal structure, consisting of periodically arranged air pores and a temperature-sensitive solvent-responsive hydrogel P (NIPAM-AM). The air pores are arranged in a face-centered hexagonal close-packed configuration. The thin film structure is divided into an upper structure 1 and a lower structure 2, both of which are composed of air pores and filling materials. The air pores are periodically stacked, and the radius of the air pores in the upper structure 1 is larger than that in the lower structure 2.

[0051] The air pore radius in the upper structure 1 is 135nm-165nm, and the air pore radius in the lower structure 2 is 105nm-145nm. In a hydrogel film, the difference in air pore radius between the upper and lower layers is at least 20nm.

[0052] The upper structure 1 has 4-7 air pore layers, and the lower structure 2 has 5-9 air pore layers. In a hydrogel film, the number of air pore layers in the upper structure 1 and the lower structure 2 are different.

[0053] Both the upper and lower structures consist of air pores and filling materials, with the filling material being P(NIPAM-AM) or other organic materials that are both solvent- and temperature-responsive.

[0054] A method for preparing a multilayer inverse opal structure hydrogel film with multi-physical quantity response includes the following steps:

[0055] Step 1: Disperse 0.04g of dried monodisperse nanospheres of the same particle size in a 10wt% trisodium citrate solution and stir with a mechanical stirrer for 3h. Then, centrifuge, sonicate, and dry the dispersion to obtain approximately 0.03g of surface-modified nanospheres. The monodisperse nanospheres are one of SiO2, TiO2, ZnO, PS, PMMA, and CdS nanospheres.

[0056] Step 2: Disperse the nanospheres obtained in Step 1 in a mixed solvent containing 55 μL of anhydrous ethanol and 0.263 ml of DMSO. Then, add 0.08 g AM, 0.4 g NIPAM, 0.01 g Irg.124 and 0.0027 g BIS to the mixed solution containing nanospheres, DMSO and anhydrous ethanol in sequence, and sonicate at 25°C for 1 h to obtain the precursor solution.

[0057] Step 3: Pour the precursor solution obtained in Step 2 into a 3D-printed rectangular box mold with a height of 0.5 mm, and then polymerize it under a UV lamp for 30 seconds to obtain a P(NIPAM-AM) opal structure hydrogel film.

[0058] Step 4: Using a rectangular box mold with a height range of 0.5 to 0.9 mm, repeat step 3 and polymerize under ultraviolet light for 30 to 60 seconds to obtain P(NIPAM-AM) opal structure hydrogel films of different thicknesses.

[0059] Step 5: Take 0.04g of dried nanospheres of different particle sizes, and repeat steps 1, 2, 3, and 4 to prepare P(NIPAM-AM) opal structure hydrogel films with different particle sizes and different film thicknesses.

[0060] Step 6: Take two P(NIPAM-AM) opal structured hydrogel films with different particle sizes and thicknesses from Step 5. Add a crosslinking agent solution between the two hydrogel films and cure them into a composite film using a UV lamp. Then, etch the composite film in an HF solution for 10 minutes to obtain a multilayer P(NIPAM-AM) inverse opal structured hydrogel film.

[0061] To investigate the temperature response, solvent response, and dual temperature and solvent response of P(NIPAM-AM) polymer hydrogels, the response process was divided into two time points: the upper layer completing the response and the entire layer completing the response.

[0062] 1. The hydrogel film was heated, and its temperature response performance was analyzed. Since NIPAM in P(NIPAM-AM) is a thermosensitive material, the temperature response range of the hydrogel film is 32–56℃ according to its material properties. Within this range, as the temperature increases, the hydrogel film gradually shrinks, and the inter-microsphere spacing gradually decreases. According to the Bragg-Snell law, the color of the hydrogel film gradually shifts to blue, which is reflected in the spectrum as the wavelength corresponding to the photonic crystal reflection peak gradually decreases. When hydrogel films with air pore radii of 145nm and 165nm in the upper structure and 145nm and 105nm in the lower structure, and with 4 and 7, 5 and 8, and 6 and 9 layers respectively, are heated to 32℃, their temperature response results are as follows: Figures 2-7 As shown.

[0063] Depend on Figures 2-7It can be seen that, without heating, the hydrogel film exhibits two reflection peaks in its reflection spectrum, corresponding to a two-photon bandgap. The color displayed by the hydrogel film is a superposition of two wavelengths of light. When the hydrogel film is heated to 32℃, firstly, as the temperature increases, the upper photonic crystal structure shrinks due to the shrinkage of the hydrogel film, reducing the inter-lattice spacing. According to the Bragg-Snell law, the reflection peak corresponding to the upper structure undergoes a blue shift. Secondly, when the entire hydrogel film is heated, in addition to the blue shift of the upper structure's reflection peak, the lower structure's corresponding reflection peak also undergoes a blue shift. Since the half-width at half-maximum (WHM) of the lower structure's main reflection peak is wider, it overlaps with the blue-shifted reflection peak of the upper structure after the blue shift, presenting a single reflection peak in the reflection spectrum. In summary, the hydrogel film exhibits two different color changes throughout the entire temperature response process, changing the previous limitation of temperature-sensitive materials to only one color change at a fixed temperature. Furthermore, it achieves a switch from a two-photon bandgap to a single-photon bandgap, i.e., a color transition from superimposed color to monochromatic color, further enriching the color modulation capabilities of hydrogel films. During the color change process, the intensity of the main reflection peak of the photonic crystal was generally greater than 0.8, reaching a maximum of 0.99. No significant loss of reflection peak intensity was observed during the temperature response of the hydrogel film. In contrast, traditional inverse opal-structured thermosensitive hydrogels, when placed at a fixed temperature, only exhibit one color change, and the reflection peak intensity is not high.

[0064] 2. Solvent response of the hydrogel film was investigated using anhydrous ethanol. Since AM in P(NIPAM-AM) is a well-responsive solvent, the hydrogel film undergoes a swelling effect when wetted with a low-surface-tension organic solvent. This swelling increases the inter-microsphere spacing, and according to the Bragg-Snell law, the hydrogel film's color shifts to red. The results of ethanol wetting responses were obtained for films with upper-layer pore radii of 145 nm and 165 nm, lower-layer pore radii of 145 nm and 105 nm, and for upper and lower layers of 4 and 7, 5 and 8, and 6 and 9 layers, as shown below. Figures 8-13 As shown.

[0065] Depend on Figures 8-13It can be seen that the initial reflection spectrum of the hydrogel film shows two reflection peaks, representing two photon band gaps. When the upper inverse opal structure is completely wetted with 30wt% anhydrous ethanol, refractive index matching occurs because the refractive index of the upper layer of the hydrogel film is similar to that of ethanol after the change. The reflection peak redshifts to the near-infrared region, so more light passes through the upper layer and reaches the lower layer. The intensity of the reflection peak corresponding to the lower layer structure is enhanced, and the color of the inverse opal structure becomes brighter, presenting a single reflection peak in the reflection spectrum. When the entire hydrogel film is wetted, refractive index matching occurs in both the upper and lower layers. Therefore, all light incident on the hydrogel film in the visible light range is transmitted, and there is no reflection peak in the reflection spectrum, making the hydrogel film transparent. The study of the wetting process of the P(NIPAM-AM) hydrogel film, through the analysis of the spectrum after the change, shows that the hydrogel film has good solvent response characteristics and can achieve switching from a two-photon band gap to a single-photon band gap, that is, the color transformation from superimposed color to monochromatic color, which can further enrich the color control of the hydrogel film. Furthermore, during the wetting response, the intensity of the main reflection peak remained above 0.82, reaching a maximum of 0.98. No significant loss of reflection peak intensity was observed during the solvent response of the hydrogel film.

[0066] 3. Solvent and temperature responsiveness studies were conducted on the hydrogel films. The P(NIPAM-AM) hydrogel films exhibited both solvent and temperature responses. When the pore radii of the upper structure were 145 nm and 165 nm, and the pore radii of the lower structure were 145 nm and 105 nm, and the number of upper and lower structure layers were 4 and 7, 5 and 8, and 6 and 9, the results of the solvent and temperature dual responses were as follows: Figures 14-19 As shown.

[0067] Depend on Figures 14-19It is known that, initially, the hydrogel film has two photonic band gaps, exhibiting a superimposed color. When the hydrogel film is heated to 32°C and simultaneously wetted with 30wt% anhydrous ethanol, the upper photonic crystal structure first shrinks with increasing temperature, reducing the interstellar spacing. According to the Bragg-Snell law, the reflection peak formed by the upper photonic crystal structure exhibits a blue shift. However, when the upper inverse opal structure is fully wetted, the reflection peak produced by the upper inverse opal structure exhibits a red shift. Combining these two effects, the reflection spectrum shows a blue shift in the hydrogel film's color. Finally, after the hydrogel film has fully responded, only one reflection peak appears in the reflection spectrum, and this peak shows a slight blue shift compared to the initial state. Considering the entire temperature and solvent dual-response process of the multilayer inverse opal structure hydrogel film, the hydrogel film produces two color changes, which can further enrich the color modulation of hydrogel films. The reflection spectrum revealed that the composite hydrogel film can respond to both temperature and solvent simultaneously, with temperature having a greater impact on the color of the hydrogel film than solvent response. During the dual-physics response, the intensity of the main reflection peak remained consistently greater than 0.9, reaching a maximum of 0.99. The intensity loss of the reflection peak during the dual-physics response of the hydrogel film was minimal.

[0068] In summary, the multilayer inverse opal hydrogel film designed in this invention can achieve dual solvent and temperature response, switching between single-photon and two-photon band gaps during the response process, further enriching the color control of the hydrogel film. Furthermore, the main reflection peak intensity in the spectrum is basically in the range of 0.8-0.99, and the hydrogel film has a bright color, greatly expanding the application prospects of structural color films in anti-counterfeiting.

Claims

1. A multilayer inverse opal hydrogel film with multi-physical quantity response, characterized in that: The structure of the film is an inverse opal photonic crystal structure, consisting of periodically arranged air pores and a temperature-sensitive solvent dual-responsive hydrogel P (NIPAM-AM). The air pores are arranged in a face-centered hexagonal close packing. The film structure is divided into an upper structure (1) and a lower structure (2), both of which are composed of air pores and filling materials. The air pores are periodically superimposed. The radius of the air pores in the upper structure (1) is larger than that in the lower structure (2). The radius of the air pores in the upper structure (1) is 135nm-165nm, and the radius of the air pores in the lower structure (2) is 105nm-145nm. The difference in the radius of the air pores between the upper and lower structures is at least 20nm. The number of air pore layers in the upper structure (1) is 4-7, and the number of air pore layers in the lower structure (2) is 5-9. The number of air pore layers in the upper structure (1) and the lower structure (2) is different.

2. A method for preparing a multilayer inverse opal hydrogel film with multi-physical quantity response as described in claim 1, characterized in that: Step 1: Prepare monodisperse nanospheres. Disperse dry monodisperse nanospheres of the same particle size in a trisodium citrate solution and stir. Then centrifuge, sonicate, and dry the stirred dispersion to obtain surface-modified nanospheres. Step 2: Disperse the surface-modified nanospheres obtained in Step 1 in a solvent containing anhydrous ethanol and DMSO. Then, add AM, NIPAM, Irg.124 and BIS to the mixture containing surface-modified nanospheres, anhydrous ethanol and DMSO in sequence, and ultrasonically disperse to obtain a precursor solution. Step 3: Pour the precursor solution obtained in Step 2 into a 3D printed rectangular box mold, and then use a UV lamp to polymerize it to obtain a P(NIPAM-AM) opal structure hydrogel film. Step 4: By repeating step 3 using rectangular box molds of different heights, P(NIPAM-AM) opal structure hydrogel films of different thicknesses can be obtained. Step 5: Take dry monodisperse nanospheres of different particle sizes and repeat steps 1, 2, 3, and 4 to prepare P(NIPAM-AM) opal structure hydrogel films with different particle sizes and different film thicknesses. Step 6: Take two P(NIPAM-AM) opal structure hydrogel films with different particle sizes of nanospheres from Step 5, and the two hydrogel films have different thicknesses. Add a crosslinking agent solution between the two hydrogel films, cure them into a composite film using a UV lamp, and then etch the composite film in a hydrofluoric acid solution to obtain a multilayer P(NIPAM-AM) inverse opal structure hydrogel film.

3. The method for preparing a multi-physical quantity responsive multilayer inverse opal hydrogel film according to claim 2, characterized in that: The monodisperse nanospheres in step 1 are one of SiO2, TiO2, ZnO, PS, PMMA and CdS nanospheres.

4. The method for preparing a multi-physical quantity responsive multilayer inverse opal hydrogel film according to claim 2, characterized in that: In step 2, the volume ratio of anhydrous ethanol to DMSO is 1:4.5-5.0, the mass-volume ratio (mg / μL) of the surface-modified nanospheres to the mixed solvent of anhydrous ethanol and DMSO is 1:9-10, and the mass ratio of the surface-modified nanospheres, AM, NIPAM, Irg.124, and BIS is 11.1-12.1:29.6-31.6:148.1-151.3:3.7-4.7:1-1.

5.

5. The method for preparing a multi-physical quantity responsive multilayer inverse opal hydrogel film according to claim 2, characterized in that: In step 1, the mass fraction of trisodium citrate solution is 10%, and the mixture is stirred for 2-4 hours. In step 2, the mixture is ultrasonically dispersed at an ambient temperature of 25°C for 1-2 hours. In step 3, the mixture is polymerized under a UV lamp for 30-60 seconds.

6. The method for preparing a multi-physical quantity responsive multilayer inverse opal hydrogel film according to claim 2, characterized in that: In step 4, the height of the 3D-printed rectangular box mold ranges from 0.5 to 0.9 mm, and in step 6, it is etched in hydrofluoric acid solution for 10 minutes.

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